bnnSurvival: Bagged k-Nearest Neighbors Survival Prediction

Implements a bootstrap aggregated (bagged) version of
the k-nearest neighbors survival probability prediction method (Lowsky et
al. 2013). In addition to the bootstrapping of training samples, the
features can be subsampled in each baselearner to break the correlation
between them. The Rcpp package is used to speed up the computation.